1. Field of the Invention
The invention generally relates to imaging devices used for the early detection of breast cancer and, more particularly, to imaging devices which use non-ionizing radiation to image the breast.
2. Description of the Prior Art
Breast cancer is one of the most frequently occurring cancers in women in the United States. At present, early detection of breast cancer offers the best hope for improving the survival rate. Mammography is currently the most reliable method for detecting breast cancer. Despite its well demonstrated usefulness, mammography has some important drawbacks: (1) ionizing radiation (x-rays) is utilized for imaging the breast and it is now widely believed that ionizing radiation may be a cause of breast cancer induction, (2) images of dense or thick breasts are difficult to obtain using mammography, and (3) mammography is unsuitable for examining young women of child bearing age because of the radiation risk.
Transillumination is a non-ionizing technique which involves, like mammography, shining radiation through the breast to create images used for detecting and localizing lesions. The photons used in transillumination are much lower in energy than the x-rays used in mammography and do not cause ionization; however, despite the safety advantage inherent with low energy photons, the photons are more easily scattered leading to a strong blurring of the images. Transillumination methods for diagnosis of breast lesions were first described in the 1920s in Cutler, Surg Gynecol, Obstet, 48:721 (1929) and have been improved with newly available technology during the last decade.
U.S. Pat. No. 4,600,011 to Watmough discloses a tele-diaphanography apparatus which operates by shining a light through the breast and detecting the transmitted light with a camera. A pseudo color generator uses the output from the camera to produce a colored image from which breast lesions are to be discerned.
U.S. Pat. Nos. 4,616,657 and 4,651,743 to Stoller show a diaphanoscopy apparatus and method for non-invasively detecting cancer in body tissues. Fiber optic cables illuminate the breast from one side and a vidicon camera detects the emerging light on the other side of the breast. The output from the camera can be viewed continuously on a video screen and also stored in a computer for further processing. The breast can be sequentially transilluminated with light having different wavelengths and the video system provides information bearing signals to data processing circuitry which determines the transmissivity at each wavelength of each point of the object within the viewing field. The breast can be viewed as a composite color image indicating the ratio of intensities of the frequencies utilized. Two ranges of light can be used for diagnosis of breast cancer. One range is intended to be sensitive to healthy tissues while the other range is intended to be more indicative of tumors.
The Lintroscan, available from Lintronics Industries of Florida, includes a video camera and computer to detect and analyze light and near infrared light beams after they have been transmitted through a breast. The system is used to detect patterns of increased vascularity which normally surround a breast cancer. Near infrared light is best for detecting the increased vascularity.
U.S. Pat. No. 4,570,638 to Stoddart et al shows a method and apparatus for spectral transmissibility examination and analysis. Recognizing that observing one frequency band may not be enough to detect tumors (especially cancerous tumors), light over the whole frequency band from 0.6 to 1.5 microns is applied to the breast using fixed light pipes to obtain an absorption pattern. The transmitted light is sent to photodetectors by another group of light pipes. A computer collects the output from the detectors for the whole frequency range and calculates a cancer index by comparing the observed absorption with that of healthy tissue.
U.S. Pat. No. 4,515,165 to Carroll discloses a device for the detection of tumors in human and animal tissue using transmission or reflection of non-ionizing radiation. A scanning mode is used to produce a shadow graph image of the tissue corresponding to the amount of absorption and scatter. Visible and infrared light is emitted from an emitter array and received by a detector array under computer control. Carroll believes that this device can differentiate between benign and malignant tumors.
The transillumination devices described above have no provisions for reducing the scattering of light. The devices rely on indirect approaches for identifying cancerous tissue such as analysis of shape, absorption patterns, or relative density. These parameters are often ill-defined because of scattering effects and make the detection of lesions by these methods unreliable. The scattering problems inherent in the design of these devices may be one reason why they are not widely used at this time.
The difficulty in observing lesions imbedded in breast tissue by transillumination is highlighted in a theoretical investigation by Navarro et al, Med Phys, 15:181 (1988). In that study, the breast is modeled as a cylinder with a diameter of 4 centimeters (cm) and a height of 4 cm filled with a homogenous scattering medium. A cylinder of 0.53 cm diameter and height filled with an absorbing material is imbedded in the homogenous scattering medium. The breast model was illuminated from the top by diffuse light. To find the distribution of light, the Boltzmann transport equation was used. To obtain a correct representation of the light distribution near absorbers and the lower boundary, the equation was solved by direct numerical method using a two dimensional geometry. The results show that the light distribution emerging from the bottom boundary of the breast is undisturbed when the small cylinder with the absorbing material is placed anywhere in the scattering medium except if it is located near the bottom border. Applying this result to imaging of breast cancers, a lesion of 0.5 cm in size will not be detected at a wavelength of 950 nm if it is located deeper than 0.5 cm from the skin surface. The Navarro et al study, in spite of its simplifying assumptions, gives a good appreciation of the relative importance of depth, size, wavelength of photons, and angle of observation on the contrast of the light image. A significant image improvement should result if scatter is reduced.
U.S. Pat. No. 4,212,306 to Mahmud discloses a breast examination device where the breast is examined by scanning a beam of light in a predetermined pattern over the breast to sequentially illuminate the entire breast. The beam of light is viewed through compression plates to permit visual detection of areas of breast tissue having lesser transparency which are suggestive of tumor growth. To enhance delineation and definition of such areas, the beam thickness (diameter) and intensity can be varied. The light beam can have a wavelength or range of wavelengths falling within the visible spectrum including the near ultraviolet and infrared regions.
U.S. Pat. No. 4,649,275 to Nelson et al discloses a high resolution breast imaging device which utilizes non-ionizing radiation of narrow spectral bandwidth. A collimated beam of light (ultraviolet, visible, or infrared) of a narrow spectral bandwidth is scanned over a breast held between compression plates which are transparent to the wavelengths of light used to image the breast. Light transmitted through the breast is recorded by photodetectors generating an analog signal which can be digitized and made available to a computer for analysis, processing and display. Collimation is used to produce a beam or beams of very small cross-section and highly directional nature such that transmitted scatter from the exit beam can be reduced. Several images can be acquired at distinct wavelengths to help differentiate normal and diseased breast materials.
U.S. Pat. No. 4,767,928 to Nelson et al discloses a high resolution breast imaging device operating under principles similar to that disclosed in U.S. Pat. No. 4,649,275 discussed above. A mask can be built in to the compression plates used for imaging. Tomographic images are obtained by rotating the light sources, collimators, and photodetectors around the perimeter of the breast. Tomographic images can also be obtained using a multiple scan beam arrangement.